The best way to acquire meteorological data is to observe sky in an aircraft. In general, civil aircrafts fly stable courses at an altitude of 8 kilometers, which is the best height between high clouds and mesospheric clouds to observe cirrus and altocumulus. In other words, the clouds at higher places than civil aircrafts are a type of cirrus, and clouds formed below them, if any, are a type of altocumulus. Because cirrus and altocumulus actually look pretty much the same, clouds must be classified by referring to sizes of cloud fraction at the ground, but it is not easy to distinguish clouds which look alike but whose fraction sizes are somewhat different.
Therefore, weather centers in countries around the world operate high altitude, long endurance (HALE) unmanned aerial vehicles (UAVs) and make them perform a variety of observations while staying at the stratosphere at an altitude of about 20 kilometers for a long time. However, a propulsion system for HALE should provide stable performance under an extreme environment of the stratosphere and its long endurance should be verified. Besides, it is also necessary to minimize harmful exhaust gas in response to global warming phenomena. Since even HALE UAVs for meteorological observations are still currently at a continuous development stage, it is true that more focuses are on aerial vehicles themselves rather than imaging technologies.
Nevertheless, given that it is difficult to control the HALE UAVs and it takes relatively more costs and maintenance fees for them to take off once, a necessity of HALE UAVs for acquiring more images effectively in a short time increases.